Distributed optical amplifying apparatus, optical communication station, optical communication system, and optical fiber cable

ABSTRACT

An optical fiber transmission line including first, second and third optical fibers connected together so that light travels through the transmission line from the first optical fiber, then through the second optical fiber and then through the third optical fiber. The first, second and third optical fibers have first, second and third characteristic values, respectively. The second characteristic value is larger than the first characteristic value and the third characteristic value. The characteristic value of a respective optical fiber being a nonlinear refractive index of the optical fiber divided by an effective cross section of the optical fiber. Pump light is supplied to the transmission line so that Raman amplification occurs in the transmission line as an optical signal travels through the transmission line.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 09/984,489,filed Oct. 30, 2001 now U.S. Pat. No. 6,721,481 now allowed.

This application claims priority to Japanese application No.2000-330966, filed Oct. 30, 2000, and which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a distributed optical amplifyingapparatus which can serve both as an optical transmission line and anoptical amplifying medium, and more particularly, to a distributedoptical amplifying apparatus which can compensate transmission loss,prevent a nonlinear optical effect, and improve an opticalsignal-to-noise ratio. Moreover, the present invention relates to anoptical fiber cable suitable for the distributed optical amplifyingapparatus, an optical communication station including the distributedoptical amplifying apparatus, and an optical communication systemincluding the distributed optical amplifying apparatus.

2. Description of the Related Art

Optical communication systems will be used in future multimedianetworks, as advances in optical communication technology should enablethe high bandwidth, high capacity, ultra long distance transmissionrequired by such future multimedia networks. Wavelength divisionmultiplexing (hereinafter abbreviated to ‘WDM’) is a significant opticalcommunication technology being developed for this purpose, as WDMeffectively utilizes the broadband characteristics and large capacity ofan optical fiber.

More specifically, in WDM optical communication systems, a plurality ofoptical signals at different wavelengths are multiplexed together into aWDM optical signal. This WDM optical signal is then transmitted througha single optical fiber as an optical transmission line. A WDM opticalcommunication system can provide extremely high bandwidth, highcapacity, long distance transmission.

In a long distance optical communication system, since a WDM opticalsignal attenuates while being transmitted through an opticaltransmission line, the WDM optical signal must be amplified after beingtransmitted a certain distance. For this reason, optical amplifyingapparatuses for amplifying the WDM optical signal are currently in use,and further research and development of such apparatuses is occurring.

Therefore, in a conventional WDM optical communication system, anoptical transmitting station uses wavelength division multiplexing tomultiplex together a plurality of optical signals at differentwavelengths into a WDM optical signal. The WDM optical signal is thentransmitted through an optical transmission line. An optical receivingstation receives the transmitted WDM optical signal from the opticaltransmission line. One or more optical repeater stations are positionedalong the optical transmission line to amplify the WDM optical signal.The number of optical repeater stations is typically determined inaccordance with system design parameters to provide a sufficient amountof amplification.

While being transmitted through the optical transmission line, the WDMoptical signal deteriorates in its waveform due to wavelengthdispersion, transmission loss, and a nonlinear optical effect.Therefore, various countermeasures have been devised.

For example, various conventional methods have been devised forproviding wavelength dispersion compensation. In one such method, adispersion-managed fiber (hereinafter abbreviated to ‘DMF’) combinesoptical fibers with different wavelength dispersion from each other.

FIGS. 1A and 1B are diagrams showing the structures of conventionaldispersion-managed fibers.

More specifically, FIG. 1A shows a partial structure between twostations in an optical communication system, where an optical repeaterstation 1004-A and an optical repeater station 1004-B are connected byan optical transmission line 1002. The optical transmission line 1002 iscomposed of an optical transmission line 1002-L1 whose wavelengthdispersion is positive and an optical transmission line 1002-L2 whosewavelength dispersion is negative. An optical signal is transmitted tothe optical repeater station 1004B from the optical repeater station1004A via the optical transmission line 1002-L1 and the opticaltransmission line 1002-L2. While being transmitted, the optical signalundergoes a positive wavelength dispersion in the optical transmissionline 1002-L1 and undergoes a negative wavelength dispersion in theoptical transmission line 1002-L2 to be compensated in a manner thataccumulated wavelength dispersion becomes almost zero. The DMF asdescribed above is disclosed, for example, in U.S. Pat. No. 5,191,631,and Japanese Patent Laid-open No. Hei 9-318824. A symmetrical DMF inwhich the wavelength dispersion is made symmetrical is also disclosed.

FIG. 1B shows a partial structure of two stations in an opticalcommunication system. An optical repeater station 1004-C and an opticalrepeater station 1004-D are connected by the optical transmission line1002. The optical transmission line 1002 is composed of an opticaltransmission line 1002-L3 whose wavelength dispersion is positive, anoptical transmission line 1002-L4 whose wavelength dispersion isnegative, and an optical transmission line 1002-L5 whose wavelengthdispersion is positive. An optical signal which is sent out from theoptical repeater station 1004-C undergoes the positive wavelengthdispersion in the optical transmission line 1002-L3, undergoes thenegative wavelength dispersion in the optical transmission line 1002-L4,and undergoes the positive wavelength dispersion in the opticaltransmission line 1002-L5. Therefore, the optical signal is transmittedto the optical repeater station 1004-D in a manner so that compensationcauses accumulated wavelength dispersion to become almost zero.Meanwhile, an optical signal sent out from the optical repeater station1004-D undergoes the positive wavelength dispersion in the opticaltransmission line 10025, undergoes the negative wavelength dispersion inthe optical transmission line 1002-L4, and undergoes the positivewavelength dispersion in the optical transmission line 1002-L3.Therefore, the optical signal is transmitted to the optical repeaterstation 1004-C in a manner so that compensation causes accumulatedwavelength dispersion to become almost zero. Such a DMF is disclosed,for example, in U.S. Pat. No. 5,778,128, a paper, “Enhanced powersolitons in optical fibers with periodic dispersion management” (N. J.Smith, F. M. Knox, N. J. Doran, K. J. Blow and I. Bennion: ElectronicsLetters, Vol. 31, No. 1, p54-p55, 4th Jan. 1996), a paper,“Energy-scaling characteristics of solitons in stronglydispersion-managed fibers” (N. J. Smith, N. J. Doran, F. M. Knox and W.Forysak: Optics Letters, Vol. 21, No. 24, p1981-p1983, 15th Dec. 1966),and a paper, “40 Gbits×16 WDM transmission over 2000 km using dispersionmanaged low-nonlinear fiber span” (Itsuro Morita, Keiji Tanaka, NoboruEdagawa and Masatoshi Suzuki: ECOC 2000, Vol. 4, p25-p26, 2000).

These conventional technologies are devised from the viewpoint ofwavelength dispersion compensation. Such technologies were not devisedin consideration of a system in which an optical transmission line alsoserves as an optical amplifying medium for distributed opticalamplification.

Meanwhile, various methods for compensating the transmission loss havealso been conventionally devised, and a distributed optical amplifyingapparatus, especially a distributed Raman amplifier, is one of them.

FIGS. 2A and 2B are diagrams showing the structures of conventional losscompensated/distributed Raman amplifiers.

FIG. 2A shows a partial structure between two stations in the opticalcommunication system described above, where the optical repeater station1004-A and an optical repeater station 1004-E are connected with theoptical transmission line 1002. In the optical repeater station 1004-E,a pump light source 1005-E for supplying pump light used for Ramanamplification is provided. The optical transmission fine 1002 iscomposed of an optical transmission line 1002-L6 which has a largeeffective cross section and an optical transmission line 1002-L7 whichhas a small effective cross section compared with that of the opticaltransmission line 1002-L6, and it is supplied with the pump light fromthe pump light source 1005-E. An optical signal is transmitted from theoptical repeater station 1004-A to the optical repeater station 1004-Evia the optical transmission line 1002-L6 and the optical transmissionline 1007-L7, and is Raman-amplified by the pump light in the opticaltransmission line 1002 while being transmitted to be compensated in sucha manner that transmission loss becomes almost zero. In other words, theoptical signal is Raman-amplified so that an output optical level of theoptical repeater station 1004-A and an input optical level of theoptical repeater station 1004-E are substantially equal to each other.The effective cross section is a part of a cross section of the opticaltransmission line in which the optical signal and the pump lightinteract with each other to cause sufficient Raman amplification. Such aDMF is disclosed, for example, in a paper, “40 Gbit/s×8 NZR WDMtransmission experiment over 80 km×5-span using distributed Ramanamplification in RDF” (R. Ohhira, Y. Yano, A. Noda, Y. Suzuki, C.Kurioka, M. Tachigori, S. Moribayashi, K. Fukuchi, T. Ono and T. Suzaki:ECOC '99, 26-30, p176-p177, September 1999, Nice, France).

Here, a size of the effective cross section correlates with a scale ofthe nonlinear optical effect. When the effective moss section is large,the nonlinear optical effect is small. On the other hand, when theeffective cross section is small, the nonlinear optical effect is large.Therefore, from the viewpoint of a choice of whether optical power inthe optical repeater station 1004-A from which the optical signal issent out is increased or optical power in the optical repeater station1004-E from which the pump light is supplied is increased, a structureas shown in FIG. 2B in also possible. In FIG. 2B, the opticaltransmission line 1002 is composed of an optical transmission line1002-L8 which has a small effective cross section and an opticaltransmission line 1002-L9 which has a large effective cross sectioncompared with the optical transmission line 1002-L8. The opticaltransmission line 1002-L8 is connected to the optical repeater station100A. Such a structure is disclosed, for example, in a paper, “Aproposal of a transmission line without any loss in a longitudinaldirection utilizing distributed Raman amplification” (Toshiaki Okuno,Tetsufumi Tsuzaki and Masayuki Nishimura: B-10-116, the 2000 SocietyConference of the Institute of Electronics, Information andCommunication Engineers).

These conventional technologies as shown in FIGS. 2A and 2B aretechnologies which are devised from the viewpoint of compensating thetransmission loss and no consideration is made for the wavelengthdispersion compensation, an optical signal-to-noise ratio (hereinafterabbreviated to ‘optical SNR’), and so on.

Furthermore, in the conventional arts as shown in FIGS. 1A and 1B andFIGS. 2A and 2B, the nonlinear optical effect, especially a nonlinearphase shift, is not taken into consideration.

It is noteworthy that the wavelength dispersion and the effective crosssection have such a correlation that an optical fiber with the positivewavelength dispersion usually has a small effective cross section and anoptical fiber with the negative wavelength dispersion usually has alarge effective cross section.

In realizing long distance transmission of an optical signal with lesserror ratio, there is a problem that the wavelength dispersion, thetransmission loss, and the nonlinear optical effect need to becompensated in a well-balanced manner as a whole instead of compensatingonly one physical quantity.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an opticalamplifying medium having appropriate characteristics for providingdistributed optical amplification and which solves various of theabove-described problems. It is also an object of the present inventionto provide a distributed optical amplifying apparatus, an optical fibercable, an optical communication station, and an optical communicationsystem which use such an optical amplifying medium.

Additional objects and advantages of the invention will be set forth inpart in the description which follows, and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

Objects of the present invention are achieved by providing a distributedoptical amplifying apparatus, including an optical fiber having a middlefield with a characteristic value which is larger than characteristicvalues of fields other than the middle field, the characteristic valueof a respective field being a nonlinear refractive index of the opticalfiber at the respective field divided by an effective cross section ofthe fiber at the respective field. The apparatus also includes a pumplight source supplying pump light to the optical fiber.

Objects of the present invention are also achieved by providing adistributed optical amplifying apparatus including a fiber line and apump light source supplying pump light to the fiber line. The fiber lineincludes first, second and third optical fibers connected together sothat light travels through the fiber line from the first optical fiber,then through the second optical fiber and then through the third opticalfiber. The first, second and third optical fibers having first, secondand third characteristic values, respectively. The second characteristicvalue is larger than the first characteristic value and the thirdcharacteristic value. The characteristic value of a respective opticalfiber being a nonlinear refractive index of the optical fiber divided byan effective cross section of the optical fiber.

Objects of the present invention are further achieved by providing adistributed optical amplifying apparatus including (a) a fiber linecomprising first, second and third optical fibers connected together sothat light traveling through the fiber line travels through the firstoptical fiber, then through the second optical fiber, and then throughthe third optical fiber, and (b) a pump light source supplying pumplight to the fiber line. A value D1/S1 of a wavelength dispersioncoefficient D1 of the first optical fiber divided by a wavelengthdispersion slope S1 thereof is almost equal to a value D2/S2 of awavelength dispersion coefficient D2 of the second optical fiber dividedby a wavelength dispersion slope S2 thereof. A sum of a value D1•L1 ofthe wavelength dispersion coefficient D1 of the first optical fibermultiplied by a length L1 thereof and a value of the wavelengthdispersion coefficient D2 of the second optical fiber multiplied by alength L2 thereof is almost zero. A wavelength dispersion coefficient, awavelength dispersion slope, and a length of the third optical fiber arealmost equal to the wavelength dispersion coefficient D1, the wavelengthdispersion slope S1, and the length L1 of the first optical fiber.Accumulated wavelength dispersion in a wavelength of an optical signaltransmitted through the fiber line is almost zero at an output of thefiber line. An accumulated wavelength dispersion slope in the wavelengthof the optical signal transmitted through the fiber line is almost zeroat the output of the fiber line.

Objects of the present invention are also achieved by providing anoptical communication station including a processing device forperforming predetermined processing for an optical signal, and a fiberline connected to the processing device. The fiber line includes first,second and third optical fibers connected together so that the opticalsignal travels through the fiber line by traveling through the firstoptical fiber, then through the second optical fiber, and then throughthe third optical fiber. The first, second and third optical fibers havefirst, second and third characteristic values, respectively. The secondcharacteristic value is larger than the first and third characteristicvalues. The characteristic value of a respective optical fiber being anonlinear refractive index of the optical fiber divided by an effectivecross section of the optical fiber. A pump light source supplies pumplight to the fiber line.

Objects of the present invention are further achieved by providing anoptical communication system including (a) an optical transmission line,(b) first and second stations connected together through the opticaltransmission line and performing predetermined processing of an opticalsignal transmitted through the optical transmission line, and (c) a pumplight source supplying pump light to the transmission line. Thetransmission line includes first, second and third optical fibersconnected together so that the optical signal travels through thetransmission line by traveling through the first optical fiber, thenthrough the second optical fiber, and then through the third opticalfiber. The first, second and third optical fibers have first, second andthird characteristic values, respectively. The second characteristicvalue being larger than the first and third characteristic values. Thecharacteristic value of a respective optical fiber is a nonlinearrefractive index of the optical fiber divided by an effective crosssection of the optical fiber.

Moreover, objects of the present invention are achieved by providing anoptical communication system including (a) first and second transmissionlines, each having first and second ends, (b) an optical transmittingstation generating an optical signal and providing the generated opticalsignal to the first end of the first transmission line so that theoptical signal travels through the first transmission line to the secondend of the first transmission line, (c) an optical repeater stationreceiving the optical signal from the second end of the firsttransmission line, amplifying the received optical signal, and providingthe amplifying optical signal to the first end of the secondtransmission line so that the amplified optical signal travels throughthe second transmission line to the second end of the secondtransmission line, and (d) an optical receiving station receiving theamplified optical signal from the second end of the second opticaltransmission line. At least one of the first and second transmissionlines includes first, second and third optical fibers connected togetherso that the optical signal travels through the respective transmissionline by traveling through the first optical fiber, then through thesecond optical fiber, and then through the third optical fiber. Thefirst, second and third optical fibers having first, second and thirdcharacteristic values, respectively, the second characteristic valuebeing larger than the first and third characteristic values. Thecharacteristic value of a respective optical fiber being a nonlinearrefractive index of the optical fiber divided by an effective crosssection of the optical fiber. Pump light source is providing to therespective transmission line.

Objects of the present invention are achieved by providing an opticalfiber cable including a plurality of optical fibers. Each optical fiberhas a characteristic value in a middle field which is larger thancharacteristic values in fields other than the middle field of theoptical fiber, the characteristic value in a respective field being anonlinear refractive index of the optical fiber in the field divided byan effective cross section of the optical fiber in the field.

Objects of the present invention are also achieved by providing anoptical fiber cable including first, second and third optical fibersconnected together so that light traveling through the cable travelsthrough the first optical fiber, then through the second optical fiberand then through the third optical fiber. The second optical fiber has anegative dispersion value. The first and third optical fibers each havea positive dispersion value. The cable optically connects together twooptical repeater stations, or an optical repeater station and an endstation.

Moreover, objects of the present invention are achieved by providing anoptical, communication system including an optical fiber cablecomprising first, second and third optical fibers connected together sothat light traveling through the cable travels through the first opticalfiber, then through the second optical fiber and then through the thirdoptical fiber. The second optical fiber has a negative dispersion value.The first and third optical fibers each have a positive dispersionvalue. The cable optically connects together (a) two optical repeaterstations with one of the optical repeater stations providing pump lightto the cable so that distributed Raman amplification occurs in thecable, or (b) an optical repeater station and an end station so that oneof the optical repeater station and the end station provides pump lightto the cable so that distributed Raman amplification occurs in thecable.

In addition, objects of the present invention are achieved by providingan apparatus including a transmission line and a pump light source. Thetransmission line includes first, second and third optical fibersconnected together from an input end to an output end of thetransmission line so that a signal light travels through the input end,then through the first optical fiber, then through the second opticalfiber, then through the third optical fiber and then through the outputend. The first, second and third optical fibers have first, second andthird characteristic values, respectively. The second characteristicvalue is larger than the first characteristic value and the thirdcharacteristic value. The characteristic value of a respective opticalfiber being a nonlinear refractive index of the optical fiber divided byan effective cross section of the optical fiber. The pump light sourcesupplies pump light to the transmission line so that the signal light isamplified by Raman amplification as the signal light travels through thetransmission line

Further, objects of the present invention are achieved by providing anapparatus including a transmission line and a pump light source. Thetransmission line includes first, second and third optical fibersconnected together from an input end to an output end of thetransmission line so that a signal light travels through the input end,then through the first optical fiber, then through the second opticalfiber, then through the third optical fiber and then through the outputend. The pump light source supplies pump light to the transmission lineso that the signal light is amplified by Raman amplification as thesignal light travels through the transmission line. A value D1/S1 of awavelength dispersion coefficient D1 of the first optical fiber dividedby a wavelength dispersion slope S1 thereof is almost equal to a valueD2/S2 of a wavelength dispersion coefficient D2 of the second opticalfiber divided by a wavelength dispersion slope S2 thereof. A sum of avalue D1•L1 of the wavelength dispersion coefficient D1 of the firstoptical fiber multiplied by a length L1 thereof and a value of thewavelength dispersion coefficient D2 of the second optical fibermultiplied by a length L2 thereof is almost zero. A wavelengthdispersion coefficient, a wavelength dispersion slope, and a length ofthe third optical fiber are almost equal to the wavelength dispersioncoefficient D1, the wavelength dispersion slope S1, and the length L1 ofthe first optical fiber. Accumulated wavelength dispersion in awavelength of the signal light is almost zero at the output end of thetransmission line. An accumulated wavelength dispersion slope in thewavelength of the signal light is almost zero at the output end of thetransmission line.

Moreover, objects of the present invention are achieved by providing anoptical communication system including (a) an optical transmission line,(b) first and second stations connected together through the opticaltransmission line and performing predetermined processing of a signallight transmitted through the transmission line, and (c) a pump lightsource supplying pump light to the transmission line so that the signallight is amplified by Raman amplification as the signal light travelsthrough the transmission line. The transmission line includes first,second and third optical fibers connected together so that the opticalsignal travels through the transmission line by traveling through thefirst optical fiber, then through the second optical fiber, and thenthrough the third optical fiber. The first, second and third opticalfibers have first, second and third characteristic values, respectively,the second characteristic value being larger than the first and thirdcharacteristic values. The characteristic value of a respective opticalfiber being a nonlinear refractive index of the optical fiber divided byan effective cross section of the optical fiber.

Objects of the present invention are also achieved by providing anoptical communication system including (a) first and second transmissionlines, each having first and second ends, (b) an optical transmittingstation generating an optical signal and providing the generated opticalsignal to the first end of the first transmission line so that theoptical signal travels through the first transmission line to the secondend of the first transmission line, (c) an optical repeater stationreceiving the optical signal from the second end of the firsttransmission line, amplifying the received optical signal, and providingthe amplifying optical signal to the first end of the secondtransmission line so that the amplified optical signal travels throughthe second transmission line to the second end of the secondtransmission line, and (d) an optical receiving station receiving theamplified optical signal from the second end of the second opticaltransmission line. At least one of the first and second transmissionlines includes first, second and third optical fibers connected togetherso that the optical signal travels through the respective transmissionline by traveling through the first optical fiber, then through thesecond optical fiber, and then through the third optical fiber. Thefirst, second and third optical fibers having first, second and thirdcharacteristic values, respectively, the second characteristic valuebeing larger than the first and third characteristic values. Thecharacteristic value of a respective optical fiber being a nonlinearrefractive index of the optical fiber divided by an effective crosssection of the optical fiber. Pump light being supplied to therespective transmission line so that the optical signal is amplified byRaman amplification as the optical signal travels through the respectivetransmission line.

Objects of the present invention are further achieved by providing anapparatus including a transmission line formed of first, second andthird optical fibers, the first optical fiber being connected to thesecond optical fiber and the second optical fiber being connected to thethird optical fiber so that a signal light traveling through thetransmission line travels through the first optical fiber, then throughthe second optical fiber, then through the third optical fiber. Thefirst, second and third optical fibers have first, second and thirdcharacteristic values, respectively, the second characteristic valuebeing larger than the first characteristic value and the thirdcharacteristic value. The characteristic value of a respective opticalfiber being a nonlinear refractive index of the optical fiber divided byan effective cross section of the optical fiber. The apparatus alsoincludes a pump light source supplying pump light to the transmissionline so that the signal light is amplified by Raman amplification as thesignal light travels through at least one of the first, second and thirdoptical fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe preferred embodiments, taken in conjunction with the accompanyingdrawings of which:

FIGS. 1A and 1B are diagrams showing the structures of conventionalwavelength dispersion-managed optical transmission lines;

FIGS. 2A and 2B are diagrams showing the structures of conventional losscompensated/distributed Raman amplifiers;

FIG. 3 is a diagram showing the structure of an optical communicationsystem, according to an embodiment of the present invention;

FIGS. 4A, 4B and 4C are a diagram showing the structure of a totallymanaged/distributed Raman amplifier and charts showing optical power andaccumulated wavelength dispersion, according to an embodiment of thepresent invention;

FIGS. 5A, 5B, 5C and 5D are charts showing the correlation betweenoptical power and transmission distance for each structure of an opticaltransmission line, according to an embodiment of the present invention;

FIG. 6 is a chart showing a state of signal optical power (an averagevalue among channels) in the optical transmission line whenRaman-amplification is performed so that input signal power of theoptical transmission line and output signal power of the opticaltransmission line become equal to each other, according to an embodimentof the present invention;

FIG. 7 is a chart showing the correlation between pump light power and aRaman on/off gain, according to an embodiment of the present invention;

FIG. 8 is a chart showing the correlation between an optical SNR and theRaman on/off gain, according to an embodiment of the present invention;

FIG. 9 is a chart showing the correlation between phase shift and theRaman on/off gain, according to an embodiment of the present invention;

FIGS. 10A and 10B are charts showing the correlation between the opticalSNR and the Raman on/off gain, according to an embodiment of the presentinvention;

FIG. 11 is a chart showing the correlation between the optical SNR andpump light power and the correlation between the phase shift and theRaman on/off gain, according to an embodiment of the present invention;

FIG. 12 is a chart showing an eye pattern of the optical transmissionline for each structure, according to an embodiment of the presentinvention;

FIGS. 13A and 13B are charts showing the correlation between the opticalSNR and the Raman or/off gain, according to an embodiment of the presentinvention;

FIGS. 14A and 14B are charts showing the correlation between the opticalSNR and the Raman on/off gain (a length ratio: approximately 0.5,transmission line length: 100 km, 50 km, respectively), according to anembodiment of the present invention;

FIGS. 15A and 15B are charts showing the correlation between the opticalSNR and the Raman on/off gain (a length ratio: approximately 1, atransmission line length: 100 km, 50 km, respectively), according to anembodiment of the present invention;

FIG. 16 is a chart showing the correlation between a relative gain andthe optical SNR (Type 2), according to an embodiment of the presentinvention;

FIG. 17 is a diagram showing the structure of an optical transmittingstation in the optical communication system, according to an embodimentof the present invention;

FIG. 18 is a diagram showing the structure of an optical repeaterstation in the optical communication system, according to an embodimentof the present invention;

FIG. 19 is a diagram shoving the structure of an optical receivingstation in the optical communication system, according to an embodimentof the present invention;

FIGS. 20A, 20B and 20C are views showing examples of mode conversionsplicing, according to an embodiment of the present invention;

FIG. 21 an explanatory chart showing a wavelength dispersion slope,according to an embodiment of the present invention; and

FIGS. 22A and 22B are diagrams showing the structure of a bi-directionaloptical communication system, according to an embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the present invention, examples of which are illustratedin the accompanying drawings, wherein like reference numerals refer tolike elements throughout.

FIG. 3 is a diagram showing the structure of an optical communicationsystem, according to an embodiment of the present invention.

FIGS. 4A, 4B and 4C are diagrams showing the structure of a totallymanaged/distributed Raman amplifier and charts showing optical signalpower and accumulated wavelength dispersion, and showing a partialstructure between two stations in the optical communication system.

Referring now to FIG. 3 and FIGS. 4A, 4B and 4C, the opticalcommunication system includes an optical transmitting station 101generating a WDM optical signal including a plurality (for example, “m”)optical signals at different wavelengths multiplexed together. Thegenerated WDM optical signal is transmitted through an opticaltransmission line 102 to an optical receiving station 103. The opticalreceiving station 103 receives the WDM optical signal and then processesthe received WDM optical signal. The optical transmission line 102receives pump light supplied by a pump light source (not shown in FIG.3, but see pump light source 105B in FIG. 4A) so that the opticaltransmission line 102 serves as an optical amplifying medium. Theoptical transmission line 102 is shown in FIG. 3 is being formed ofindividual optical transmission lines 102-1 through 102-a+1, and isshown in FIG. 4A as being formed of individual optical transmissionlines 102-L1, 102-L2 and 102-L3.

Optical repeater stations 104 (shown individually as optical repeaters104-1 through 104 a in FIG. 3, and 104-A and 104-B in FIG. 4A) areconnected between each of the optical transmission lines 102 in theoptical communication system. Plural optical repeater stations 104 aredisposed between each of the individual optical transmission linesforming optical transmission line 102 according to need. Each opticalrepeater station 104 includes a pump light source (such as the pumplight source 105B shown for optical repeater station 104-B in FIG. 4A)for supplying the pump light used for distributed optical amplificationto the optical transmission line 102. Furthermore, each optical repeaterstation 104 would typically have a concentrated optical amplifying partand/or an optical add/drop part according to need. The concentratedoptical amplifying part is, for example, an optical circuit foramplifying the WDM optical signal to a predetermined output opticallevel. The optical add/drop part is, for example, an optical circuit foradding/dropping an optical signal corresponding to a predeterminedchannel to/from the WDM optical signal. Moreover, a pump light source(such as pump light source 105-B in FIG. 4A) is also disposed in theoptical receiving station 103.

The optical transmission line 102 is an optical fiber in which, for eachsection between repeaters, or between a repeater and an end station, hasa characteristic value in its middle field that is larger thancharacteristic values in fields other than the middle field, where thecharacteristic value is a nonlinear refractive index n₂ divided by aneffective cross section A_(eff). More specifically, as shown in FIG. 4A,the optical transmission line 102 is comprised of a first opticaltransmission line 102-L1 which has a predetermined characteristic value,a second optical transmission line 102-L2 which is connected to thefirst optical transmission line 102-L1 and has a characteristic valuelarger than that of the first optical transmission line 102-L1, a thirdoptical transmission line 102-L3 which is connected to the secondoptical transmission line 102-L2 and has a characteristic value smallerthan that of the second optical transmission line 102-L2. The firstoptical transmission line 102-L1 is connected to an optical repeaterstation 104-A (sometimes, for example, to the optical transmittingstation 101 in FIG. 3) disposed on an upstream side of the transmissiondirection of the WDM optical signal. The third optical transmission line102-L3 is connected to an optical repeater station 104-B (sometimes, forexample, to an optical receiving station 103 in FIG. 3) disposed on adownstream side of the transmission direction of the WDM optical signal.

In the optical communication system as described above, an opticalsignal is Raman-amplified in the optical transmission line 102 by thepump light which is supplied from a pump light source so thattransmission loss of the optical transmission line 102 can becompensated. Such pump light might be provided, for example, from a pumplight source inside of an optical repeater station 104 (such as pumplight source 105-B in FIG. 4A) and/or by a pump light source in anoptical receiving station 103.

A Raman amplification characteristic can be calculated by the followingformula 1.$\frac{\mathbb{d}{P_{f}\left( {z,\upsilon} \right)}}{\mathbb{d}z} = {{{- \alpha}\quad(\upsilon){P_{f}\left( {z,\upsilon} \right)}} + {{\gamma(\upsilon)}{P_{b}\left( {z,\upsilon} \right)}} + {\int_{\xi > \upsilon}{\left\{ {{\frac{g\quad r}{A_{eff}}{\left( {\upsilon - \xi} \right)\left\lbrack {{P_{f}\left( {z,\xi} \right)} + {P_{b}\left( {z,\xi} \right)}} \right\rbrack}{P_{r}\left( {z,\upsilon} \right)}} + {2h\quad\upsilon\quad\frac{g\quad r}{A_{eff}}{\left( {\upsilon - \xi} \right)\left\lbrack {{P_{r}\left( {z,\xi} \right)} + {P_{b}\left( {z,\xi} \right)}} \right\rbrack} \times \left( {1 + \frac{1}{{\mathbb{e}}^{{h{({\xi - \upsilon})}}/{kT}} - 1}} \right)}} \right\}{\mathbb{d}\xi}}} - {\int_{\xi < \upsilon}{\left\{ {{\frac{g\quad r}{A_{eff}}{\left( {\upsilon - \xi} \right)\left\lbrack {{P_{f}\left( {z,\xi} \right)} + {P_{b}\left( {z,\xi} \right)}} \right\rbrack}{P_{f}\left( {z,\upsilon} \right)}} + {2h\quad\upsilon\quad\frac{g\quad r}{A_{eff}}{\left( {\upsilon - \xi} \right)\left\lbrack {{P_{f}\left( {z,\xi} \right)} + {P_{b}\left( {z,\xi} \right)}} \right\rbrack} \times \left( {1 + \frac{1}{{\mathbb{e}}^{{h{({\xi - \upsilon})}}/{kT}} - 1}} \right)}} \right\}{\mathbb{d}\xi}}}}$

Here, when a z-axis is taken parallel to the transmission direction inthe optical transmission line 102, P_(f) (z, υ) is power of forwardlight (all light propagating forward along the optical transmissionline) at a distance z and a frequency υ, P_(b) (z, υ) is power ofbackward light (all light propagating backward along the opticaltransmission line) at the distance z and the frequency υ, α (υ) isattenuation amount at the frequency υ, γ (υ) is a Reyleigh's scatteringcoefficient at the frequency υ, gr (Δυ) gr (ξ−υ) is a Raman's gaincoefficient between a frequency ξ and υ, A_(eff) is the effective crosssection of the optical transmission line 102, h is a Planck's constant,k is a Boltzmann's coefficient, and T is temperature of the opticaltransmission line 102.

For formula 1, a paper, “Pump interactions in a 100-nm Bandwidth Ramanamplifier” (H. Kidorf, K. Rottwitt, M. Nissov, M. Ma, and E.Rabarijaona: IEEE Photonics Technology Letters, Vol. 11, No. 5,p530-p532) is referred to.

By setting optical power of the pump light properly according to theformula 1, an input level on a receiving side and an output level on atransmitting side can be made substantially equal to each other, asshown in FIG. 4B, in the section between the optical transmittingstation 101 and the optical repeater station 104-1, between each of theoptical repeater stations 104 (for example, between optical repeaterstations 104-1 and 104-2, and between optical repeater stations 104-a−1and 104 a), and between the optical repeater station 104-a and theoptical receiving station 103.

Alternately, the input level on the receiving side and the output levelon the transmitting side can be made substantially equal not accordingto formula 1, but by detecting the output level at a transmitting end,notifying the receiving side of this value, and controlling the power ofthe pump light while detecting the input level at a receiving end.

Note that a horizontal axis indicates distance and a vertical axisindicates optical signal power in FIG. 4B.

A nonlinear phase shift φNL is given by the following formula 2.$\begin{matrix}{\Phi_{NL} = {k{\int_{O}^{L}{\frac{n_{2}(z)}{A_{eff}(z)}{P(z)}{\mathbb{d}z}}}}} & \text{(Formula~~2)}\end{matrix}$

Here, n₂ (z) is a nonlinear refractive index regarding a direction of az-axis, A_(eff) (z) is an effective cross section regarding the z-axis,P (z) is optical power regarding the z-axis, k is expressed as k=1/λ,and λ is a signal wavelength.

By formula 2, the nonlinear phase shift can be prevented by making thecharacteristic value small in a field with large optical power, forexample, in a field near the transmitting end where the optical signalis sent out and a field near the receiving end where the pump light issupplied. Furthermore, in a field with small power, for example, in afield near a middle of the optical transmitting line 102 in thetransmission direction, even when the characteristic value is increased,the nonlinear phase shift does not increase very much.

Therefore, the optical transmission line 102 as described above, wherethe characteristic value of the middle optical transmission line 102-L2is made larger than the characteristic values of the opticaltransmission lines 102-L1 and 102-L3 on both sides, can prevent thenonlinear phase shift.

Moreover, from the viewpoint of wavelength dispersion of the opticaltransmission line 102, the correlation between the accumulatedwavelength dispersion (ps/nm) and the distance (km) is such as shown inFIG. 4(C) when, for example, the first optical transmission line 102-L1and the third optical transmission line 102-L3 have positive wavelengthdispersion (+D), and the second transmission line 102-L2 has negativewavelength dispersion (−D). More specifically, accumulated wavelengthdispersion of the optical communication system is depicted as awavelength dispersion diagram where the wavelength dispersion increasesbetween 0 and L1 as the distance increases, the wavelength dispersiondecreases between L1 and L1+L2 as the distance increases, and thewavelength dispersion increases again between L1+L2 and L1+L2+L3 as thedistance increases. Therefore, the optical transmission line 102 canalso compensate the wavelength dispersion.

In addition, compared with a two-division structure of a DMF, deviationamount of the accumulated wavelength dispersion in the opticaltransmission line can be reduced and the accumulated wavelengthdispersion averaged over the distance can be made small so that waveformdeterioration due to a nonlinear optical effect can be reduced.

Incidentally, as for the wavelength dispersion of the opticaltransmission line 102, it is also suitable that the first opticaltransmission line 102-L1 and the third optical transmission line 102-L3have the negative wavelength dispersion (−D) and the second opticaltransmission line 102-L2 has the positive wavelength dispersion (+D).

Here, supposing that a wavelength dispersion coefficient, a wavelengthdispersion slope, and a length of the first optical transmission line102-L1 are D1, S1, and L1 respectively, and a wavelength dispersioncoefficient, a wavelength dispersion slope, and a length of the secondoptical transmission line 102-L2 are D2, S2, and L2 respectively, and awavelength dispersion coefficient, a wavelength dispersion slope, and alength of the third optical transmission line 102-L3 are D3, S3, and L3respectively, when the optical transmission line is structured tosatisfy the correlation in the following formula, the wavelengthdispersion slope is compensated at the same time, so that uniformtransmission characteristics can be realized in respective signalwavelengths.D1/S1=D2/S2=D3/S3, D1·L1+D2·L2+D3·L3=0  (Formula 3)

Note that a horizontal axis indicates the distance and a vertical axisindicates the accumulated wavelength dispersion in FIG. 4C.

Thus, the optical communication system as shown in FIG. 3 and FIGS. 4A,4B and 4C can compensate the transmission loss and the wavelengthdispersion and prevent the nonlinear phase shift to greatly improve anoptical SNR, as compared with a conventional optical communicationsystem. This makes it possible to lengthen the transmission distancecompared with that in a conventional optical communication system.

Simulations to certify the above-described effects are explained next.

FIGS. 5A, 5B, 5C and, 5D are charts showing the correlation between theoptical power and the transmission distance for each structure of theoptical transmission line.

The simulations are carried out in an optical communication systemhaving a DMF, which serves as an optical amplifying medium and anoptical transmission line, and an optical repeater station which isconnected to the DMF, with the structure of the DMF being changed. Pumplight is supplied to the DMF via backward pumping, and an erbium-dopedoptical fiber amplifier with a noise figure 6 dB is provided in theoptical repeater station.

The simulations were carried out for three typical structures of theDMF. A DMF in a first case (Type 1) is composed of a first optical fiberwith +D and a second optical fiber with −D and it is a case where thepump light is supplied to the second optical fiber. A DMF in a secondcase (Type 2) is composed of a first optical fiber whose characteristicvalue n₂/A_(eff) is small compared with that of a second optical fiberand which has positive wavelength dispersion, the second optical fiberwhich has negative wavelength dispersion, and a third optical fiberwhose characteristic value n₂/A_(eff) is small compared with that of thesecond optical fiber, and it is a case where the pump light is suppliedto the third optical fiber. Incidentally, in this simulation, the firstoptical fiber and the third optical fiber use a fiber with the samecharacteristic and their lengths are equal to each other. A DMF in athird case (Type 3) is composed of a second optical fiber with −D and afirst optical fiber with +D, and it is a case where the pump light issupplied to the first optical fiber with +D.

The second case (Type 2) is the optical transmission system according tothe present invention.

The characteristic parameters of the first to the third optical fibersare shown in Table 1. These characteristic parameters are examples usingvalues which can be realized easily from the viewpoint of manufacturingand other values may of course be used. The characteristic parameters inTable 1 are values at 1550 nm.

TABLE 1 Characteristic parameters of first optical fiber and secondoptical fiber (L1 + L3:L2 = 2:1) Effective Cross Nonlinear Reyleigh'sWavelength section Raman's gain Refractive Scattering Optical Length LDispersion D Loss α Aeff coefficient gr index n₂ Coefficient γ Fiber(km) (ps/nm/km) (dB) (μm²) (m/W) (W⁻¹) (m⁻¹) first L1 + L3 = 68.7 +200.174 110 0.56 × 10⁻¹³ 2.9 × 10⁻²⁰ 5.8 × 10⁻⁸ second L2 = 31.3 −43.90.274 20 0.56 × 10⁻¹³ 4.0 × 10⁻²⁰  25 × 10⁻⁸ 100.0

As shown, for example, in FIG. 3, the total length of the DMF is, forexample, 100 km in all the cases (Type 1, Type 2, and Type 3). Accordingto the characteristic parameters in Table 1, the total loss of theoptical fibers of 100 km is 20.5 dB. In each of the first to the thirdcases (Type 1, Type 2, and Type 3), the lengths of the first opticalfiber and the second optical fiber, as shown in FIG. 5A, are adjusted ina manner that accumulated wavelength dispersion becomes zero at 1550 nmafter a WDM optical signal is transmitted through the DMF (that is,after transmitted for 100 km). In the WDM optical signal, wavelengths of44 waves are multiplexed at an interval of 100 GHz at 1529 nm to 1569 nmof a C band. An excitation wavelength of the pump light is set at awavelength at which the WDM optical signal is Raman-amplified based onthe (formula 1) and optical power of the pump right is set in a mannerthat a deviation of optical power between channels at an output end ofthe DMF is within±0.2 dB.

Note that a horizontal axis indicates distance and a vertical axisindicates the accumulated wavelength dispersion in FIG. 5A. FIG. 5Cshows a state of signal optical power (an average value among channels)in the optical transmission line when Raman-amplification is performedso that input signal power in the optical transmission line and outputsignal power in the optical transmission line become equal to eachother. FIG. 5C shows a case when the transmission distance is 100 km andFIG. 5D shows a case when the transmission distance is 50 km.

The results are shown in FIG. 6 to FIG. 11.

FIG. 6 is a chart showing a state of the signal optical power (theaverage value among th channels) in the optical transmission line whenthe Raman-amplification is performed so that input signal power in theoptical transmission line and the output signal power in the opticaltransmission line become equal to each other.

FIG. 7 is a chart showing the correlation between the pump light powerand a Raman on/off gain.

FIG. 8 is a chart showing the correlation between an optical SNR and theRaman on/off gain.

FIG. 9 is a chart showing the correlation between a phase shift and theRaman on/off gain.

FIGS. 10A and 10B are charts showing the correlation between the opticalSNR and the Raman on/off gain.

FIG. 11 is a chart showing the correlation between the optical SNR andthe pump light power and the correlation between the phase shift and theRaman on/off gain.

Here, FIG. 7 to FIGS. 10A and 10B are the results when the transmissiondistance is 100 km.

Note that in FIG. 6, a horizontal axis indicates the distance shown in aunit of km and a vertical axis indicates the optical power in theoptical transmission line 102 shown in a unit of dBm. In FIG. 7, a lowerhorizontal axis indicates the Raman on/off gain shown in the unit of dBand a vertical axis indicates the pump light power shown in the unit ofdBm. In FIG. 8, a lower horizontal axis indicates the Raman on/off gainshown in the unit of dB and a vertical axis indicates the optical SNRshown in the unit of dB. In FIG. 9, a lower horizontal axis indicatesthe Raman on/off gain shown in the unit of dB and a vertical axisindicates the phase shift. In FIGS. 10A and 10B, each lower horizontalaxis indicates the Raman on/off gain shown in the unit of dB and eachvertical axis indicates the optical SNR shown in the unit of dB. In FIG.7 to FIGS. 10A and 10B, each upper horizontal axis indicates a relativegain. Here, the relative gain is defined as a value of the Raman on/offgain (dB) divided by total transmission loss (dB), and in thissimulation, the total transmission loss is 20.5 dB. In FIG. 11, ahorizontal axis indicates the pump light power shown in a unit of dBmand a vertical axis indicates the optical SNR shown in a unit of dB. Ineach of the drawings, the results for the first case (Type 1) areindicated with ● or ∘, the results for the second case (Type 2) areindicated with ▪ or □, and the results for the third case are indicatedwith ▾ or Δ. The optical SNR is calculated by the following formula 4.Optical SNR=(Optical SNR _(DRA) ⁻¹+Optical SNR _(EDFA) ⁻¹)⁻¹  (Formula4)

Here, an optical SNR_(DRA) is an optical SNR by the Raman amplificationand an optical SNR_(EDFA) is an optical SNR by the erbium-doped opticalfiber amplifier.

The Raman on/off gain is a ratio between an input level at a receivingend when the pump light is supplied (on) and an input level at thereceiving end when the pump light is not supplied (off).

FIG. 6 is a chart in which each of the charts in FIG. 5C is summarizedin one chart. As is clear from FIG. 6, an extent of decrease in anoptical level in the transmission distance is the lowest in the secondcase (Type 2) when transmission is performed. In a 0 km to approximately50 km distance, a characteristic of each of the cases is substantiallythe same with each other and in an approximately 50 km to 100 kmdistance, the characteristic of each of the cases is different from eachother to a great extent. Therefore, it is dear that the Ramanamplification is performed remarkably in a distance up to approximately50 km from a receiving end to which the pump light is supplied.

As is dear from FIG. 7, the pump light power required for obtaining thesame Raman on/off gain increases in the order of the first case (Type1), the second case (Type 2), and the third case (Type 3). In otherwords, the Raman on/off gain can be obtained with the smallest pumpfight power in the first case, which is the most efficient. This isbecause a −D fiber whose nonlinear effective cross section is small anda Raman gain coefficient is large is allotted in a field closest to apump right incident end (that is, a field where the pump light power issufficiently large). Note that a case where the Raman on/off gain is20.5 dB in FIG. 7 is shown in FIG. 6. More specifically, in order tomake an input level at the receiving end almost equal to an output levelat a transmitting end, the pump light power supplied in each of thecases is different from each other.

Meanwhile, when the Raman on/off gain is the same, the optical SNRdeteriorates in the order of the second case (Type 2), the third case(Type 3), and the first case (Type 1) as is dear from FIG. 8.

In a transmission system utilizing optical amplification, the opticalSNR is an essential factor in determining signal quality. In FIG. 8,solid lines indicate the optical SNRs defined by a ratio Ps/Pn of signallight power Ps to noise light power Pn (a definition of the optical SNRwhich is generally used). Here, the noise light power Pn is a sumPn=Pr+Pe of spontaneous Raman scattered light power Pr accompanying theRaman amplification and amplified spontaneous emission (ASE) power Pe bythe erbium-doped optical fiber amplifier (EDFA). Furthermore, in thetransmission system utilizing the Raman amplification, cross talk due todouble Reighleigh's scattering deteriorates signal quality. Therefore,in this study, an effect caused by the double Reighleigh's scattering isalso studied. Broken lines in FIG. 8 indicate the optical SNRs when thedouble Reighlegh's scattering is taken into consideration. In thiscalculation, the double Reighlegh's scattered light is also taken as thenoise light besides the spontaneous Raman scattered light and the ASE.More specifically, the noise light power Pn is defined by a sumPn=Pr+Pe+Pd of the spontaneous Raman scattered light power Pr, the ASEpower Pe, and double Reighleigh's scattered light power Pd, and theoptical SNR is defined by the ratio Ps/Pn of the signal optical power Psto the noise optical power Pn.

When the Raman on/off gain is the same, the nonlinear phase shiftworsens in the order of the first case (Type 1), the second case (Type2), and the third case (Type 3) as is dear from FIG. 9.

Therefore, in compensating the transmission loss, the second case (Type2) is the most preferable when considering a balance among efficiency ofthe Raman on/off gain, improvement of the optical SNR and prevention ofthe nonlinear phase shift.

FIGS. 10A and 10B and FIG. 11 are charts in which this is seen moreclearly.

FIGS. 10A and 10B are results when input power of the optical fiber isadjusted so that the nonlinear phase shift becomes a reference valueunder each condition. The reference value is a value of the nonlinearphase shift which occurs in the first case (Type 1) under the conditionthat an input level is −2 dBm and the pump light is not supplied.

Referring to calculation results (solid lines in FIG. 10A) when aninfluence of a Reyleigh's cross talk is taken into consideration, it isclear that the highest (the most preferable) optical SNR is obtained inthe case of Type 2. It is also clear that the optical SNR in the case ofType 2 is higher than that in the other cases (Type 1 and 3) in a rangewhere the relative gain is approximately 0.5 or more (the Raman on/offgain is approximately 10 dB or more).

Furthermore, referring to calculation results (dotted lines in FIG. 10B)where the Reyleigh's cross-talk is taken into consideration, it is clearthat the highest optical SNR obtained in the case of Type 2 is highcompared with the highest optical SNRs in the other cases (Type 1 andType 3). Therefore, the result that the most preferable optical SNR canbe obtained in the case of Type 2 is also true to the case when theReyleigh's cross-talk is taken into consideration. It is also clear thatthe optical SNR in the case of Type 2 is high compared with those in theother cases (Type 1 and 3) in a range where the relative gain isapproximately 0.5 to 1 (the Raman on/off gain is approximately 10 to20.5 dB).

FIG. 11 is a chart where the horizontal axis in FIGS. 10A and 10B isreplaced by that indicating the pump light power instead and thenonlinear phase shift is also plotted. Solid lines in FIG. 11 indicatethe nonlinear phase shifts and broken lines indicate the optical SNRs.

At present, an upper limit value of the pump light power is limited ataround +27 to +30 dBm from a safety point of view and by a maximumoutput of the pump light source. However, in Type 2, it is dear that apreferable optical SNR can be obtained without exceeding the upper limitvalue of the pump light power.

Thus, when considering a balance among compensation of the transmissionloss, the efficiency of the Raman on/off gain, and the prevention of thenonlinear phase shift, the second case (Type 2) is the most preferable.Furthermore, since the deviation amount of the accumulated wavelengthdispersion in the optical transmission line can be reduced in the secondcase (Type 2), as shown in FIG. 4C and FIG. 5A, it is the best from theviewpoint of dispersion compensation. In order to confirm this, thefollowing transmission waveform simulation is carried out

FIG. 12 is a chart showing an eye pattern for each structure of theoptical transmission line.

FIG. 12 is a chart showing the eye pattern after a return zero (RZ)optical signal of 40 Gbit/S with each signal being disposed at aninterval of 100 GHz is input into the optical transmission line of eachof the first to the third case (Type 1, Type 2, and Type 3) at an inputlevel of +4 dBm/ch. and transmitted for 600 km (100 km×6 span).

As is clear from FIG. 12, the second case (Type 2) according to thepresent invention can obtain the most preferable wavelength with thewidest eye pattern aperture.

Therefore, the optical transmission line in the second case (Type 2)compensates the wavelength dispersion, the transmission loss, and thenonlinear optical effect in a well-balanced manner as a whole and alsoimproves the optical SNR most.

Incidentally, in the above description, explanation is given about thecase where a ratio L2/(L1+L3) of the length of the second optical fiber(L2) to the total length of the first optical fiber (L1+L3) isapproximately 0.5, but other ratios are permissible. The length ratio isstudied below.

According to reference papers, catalog investigation, and logicalstudies, values of the characteristic parameters to be taken by opticalfibers which can be manufactured with the present technology are shownin Table 2. Here, the characteristic parameters in Table 2 are values ata wavelength of 1550 nm.

TABLE 2 Characteristic parameters of feasible optical fiber EffectiveNonlinear Cross Refractive Slope of Section Index Wavelength WavelengthOptical Aeff n₂ n₂/Aeff Dispersion D Dispersion Fiber (μm²) (W⁻¹)(1/m⁻¹) (ps/nm/km) (ps/nm²/km) first 55˜120 2 × 10⁻²⁰˜3 × 10⁻²⁰ 1.7 ×10⁻¹⁰˜5.5 × 10⁻¹⁰ D1 = +8˜+21   0.05˜0.07 second 15˜45 2 × 10⁻²⁰˜4 ×10⁻²⁰ 4.4 × 10⁻¹⁰˜26.7 × 10⁻¹⁰ D2 = −14˜−60 −0.03˜−0.3

In order to make the accumulated wavelength dispersion zero in onesection (between two stations), the ratio L2/(L1+L3) of the secondoptical fiber length L2 to the total length (L1+L3) of first opticalfiber is required to be made as follows:L2/(L1+L3)=−D1/D2

It is clear that the following formula holds good in the characteristicparameters of the optical fiber shown in Table 2.L2/(L1+L3)=−D1/D2=0.17˜1.5

Furthermore, according to the results as shown in FIG. 5C and so on, inorder to obtain the Raman amplification characteristic for compensatingthe transmission loss with smaller pump light power, it is moreadvantageous to lengthen the length L2 of the second optical fiber, buton the other hand, in order to prevent the nonlinear phase shift, it ismore advantageous to shorten the length L2 of the second optical fiber.Therefore, from the viewpoint of this balance, it is preferable that theratio L2/(L1+L3) of the second optical fiber length L2 to the totallength (L1+L3) of the first optical fiber approximately 0.5 to 1.

In order to study this in detail, the following simulation is carriedout. It is described above that the present invention is effective whenthe length ratio is approximately 0.5, and therefore, a case where thelength ratio is increased to 1 is explained below. The characteristicparameters of the optical fiber in this case are shown in Table 3.

TABLE 3 Characteristic parameters of first optical fiber and secondoptical fiber (L1 + L3:L2 = 1:1) Effective Raman's Nonlinear Cross GainRefractive Reyleigh's Wavelength Section Coefficient Index ScatteringOptical Length L Dispersion D Loss α Aeff gr n₂ Coefficient γ Fiber (km)(ps/nm/km) (dB) (μm²) (m/W) (W⁻¹) (m⁻¹) first L1 + L3 = 50 +14 0.178 1100.56 × 10⁻¹³ 2.7 × 10⁻²⁰ 5.8 × 10⁻⁸ second L2 = 50 −14 0.230  40 0.56 ×10⁻¹³ 3.3 × 10⁻²⁰  15 × 10⁻⁸ 100.0

The results of the simulation are shown in the drawings.

The results of the simulation are shown in FIGS. 13A and 13B. FIGS. 13Aand 13B are charts showing the correlation between the optical SNR andthe Raman on/off gain and shows results when the input power of theoptical fiber is adjusted so that the nonlinear phase shift becomes areference value. The reference value is a value of the nonlinear phaseshift which occurs in the Type 1 structure under the condition where theinput level of the optical fiber is −2 dBm/ch. and the pump light is notsupplied. Referring to the calculation result (solid lines in FIGS. 13Aand 13B) when the influence of the Reyleigh's cross talk is not takeninto consideration, it is dear that the highest (the most preferable)optical SNR can be obtained in the case of Type 2. Moreover, the opticalSNR in the case of Type 2 is high compared with those in the other cases(Type 1 and 3) in the range where the relative gain is approximately 0.5or more (the Raman on/off gain is approximately 10 dB or more).

Furthermore, referring to the calculation result when the Reyleigh'scross talk is taken into consideration (dotted lines in FIGS. 13A and13B), it is dear that a maximum optical SNR obtained in the case of Type2 is high compared with maximum optical SNRs in the other cases (Type 1and 3). Therefore, the result that the most preferable optical SNR canbe obtained in the case of Type 2 is also true when the Reyleigh's crosstalk is taken into consideration. In addition, the optical SNR in thecase of Type 2 is high compared with those in the other cases (Type 1and 3) in the range where the relative gain is approximately 0.5˜1 (theRaman on/off gain is approximately 10˜20.5 dB).

The results described above certify that the present invention is alsoeffective when the length ratio of the optical fiber L2/(L1+L3) isapproximately 1 instead of 0.5. Therefore it can be said that thepresent invention is especially effective when the length ratioL2/(L1+L3) is 0.5˜1.

Next a length of the transmission line for which the present inventionis effective is studied. In order to obtain the Raman amplificationcharacteristic which compensates the transmission loss with less pumplight, it is more advantageous to shorten the total length of thetransmission line (L1+L2+L3), but when the length of the transmissionline is shortened, signal optical power in the second optical fiber witha large nonlinear coefficient increases so that the effects of thepresent invention tend to lower. In order to confirm this, simulationsare carried out with the length of the transmission line being shortenedup to 50 km.

FIGS. 14A and 14B show the optical SNR when the length ratio (L1+L3) isapproximately 0.5 and the length of the transmission line is 100 km and50 km, respectively. FIGS. 15A and 15B show the optical SNR when thelength ratio L2/(L1+L3) is approximately 1 and the length of thetransmission line is 100 km and 50 km, respectively. FIGS. 14A and 14Band FIGS. 15A and 15B are both results when the influence of the doubleReyleigh's scattering is taken into consideration. As is clear from thedrawings, the optical SNRs in the cases of Type 2 and Type 1 showpreferable characteristics compared with that of Type 3, but since themaximum optical SNRs of Type 1 and Type 2 are almost at the same level,it can be seen that the superiority of Type 2 (or the present invention)over the other types is lowered compared with that in the case of the100 km length of the transmission line. Therefore, it is dear that thepresent invention is especially effective when the length of thetransmission lin is 50 km or more instead of shortening it excessively.However, when it is 50 km or less, the superiority of the presentinvention from the viewpoint of the dispersion compensation cablemanufacturing efficiency is not lost.

FIG. 16 is a chart in which the results described above are summarizedwith regard to the correlation between the relative gain and the opticalSNR in the case of Type 2. All of the curves show the results when thedouble Reyleigh's scattering is taken into consideration and thenonlinear phase shift is fixed. According to FIG. 16, it is clear thatin all of the cases, the preferable optical SNR can be obtained in therange in which the relative gain is 0.5 to 1.

Next, example structures of the optical transmitting station 101, theoptical repeater station 104, and the optical receiving station 103 aredescribed in detail.

First, the structure of the optical transmitting station 101 isexplained.

FIG. 17 is a diagram showing the structure of the optical transmittingstation in the optical communication system. In FIG. 17, optical senders(hereinafter abbreviated to ‘OS’) 211-1 through 211-m each generates anoptical signal which is input to an optical multiplexer (hereinafterabbreviated to ‘MUX’) 222. The number of optical senders would typicallycorrespond to the number of wavelengths to be multiplexed together intoa WDM optical signal. For example, m optical senders OS 211-1 through OS211-m would be provided to generate m optical signals, respectively, atdifferent wavelengths. These m optical signals are then multiplexed byMUX 222 into a WDM optical signal.

Each OS 211-1 through 211-m might comprise, for example, a semiconductorlaser for oscillating laser light at a predetermined wavelength, anexternal modulator such as, for example, a Mach-Zehnder interferometertype optical modulator for modulating the laser light by information tobe transmitted to thereby generate a resulting optical signal, and asemiconductor optical amplifier for amplifying the optical signal. Anoscillation wavelength of the semiconductor laser is set according toeach channel of the WDM optical signal. For example, a WDM opticalsignal in a C band is arranged between 1530 nm˜1570 nm. Incidentally,there are other bands such as an S+ band (1450 nm˜1490 nm), an S band(1490 nm˜1530 nm), an L band (11570 nm˜1610 nm), and an L+ band (1610nm˜1650 nm). Of course, an optical sender is not limited to thisspecific embodiment, and many different types and configurations ofoptical senders are readily available.

The MUX 222 multiplexes the wavelengths of respective optical signalsoutput from OSs 221-1˜221-m, to thereby output a WDM optical signal.

The WDM optical signal output from the MUX 222 is input to an opticalcoupler (hereinafter abbreviated to ‘CPL’) 223 to be divided, forexample, into two signals. One of the divided signals is input to anoptical amplifying part 240 while the other is input to a photodiode(hereinafter abbreviated to ‘PD’) 226.

The CPL 223 is an optical component for dividing incident light into twosignals to thereby output the two signals. For example, a microopticoptical coupler such as a half mirror, an optical fiber type opticalcoupler of a fused fiber, an optical waveguide type optical coupler,etc., are examples of possible devices which can be sued as CPL 223.However, the present invention is not limited to these examples.

A PD 226 is a photoelectric converter for generating current accordingto optical power of received light. An output of the PD 226 is input toan analog-digital converter (hereinafter abbreviated to ‘A/D’) 228 forconverting an analog input to a digital output. An output of the A/D 228is input to a central processing unit (hereinafter abbreviated to ‘CPU’)231 such as a microprocessor for computing and processing. An inputlevel of the WDM optical signal which is input to the optical amplifyingpart 240 is detected by the PD 226.

The optical amplifying part 240 is a concentrated optical amplifyingcircuit and comprises, for example, CPLs 241 and 243, an erbium-dopedoptical fiber (hereinafter abbreviated to ‘EDF’) 242, laser diodes(hereinafter abbreviated to ‘LD’) 244 and 245, and an LD drive circuit246. An erbium element is one of rare earth elements, whose elementsymbol is Er and atomic number is 68. Elements which belong tolanthanoids have similar properties to each other.

The WDM optical signal input to the optical amplifying part 240 is inputto the CPL 241. To the CPL 241, laser light output from the LD 244 isalso input as pump light for the EDF 242. Various semiconductor lasers,for example, a Fabry-perot-resonating type laser, a distributed feedbacklaser, a distributed brag reflector laser, etc., can be utilized for theLD. However, the present invention is not limited to these examples.

The CPL 241 multiplexes the WDM optical signal input to the opticalamplifying part 240 and the laser light output from the LD 244 andthereafter, inputs it to the EDF 242. To another end of the EDF 242laser light output from the LD 245 is also input via the CPL 243.

The EDF 242 absorbs the laser light output from the LD 244 and the LD245 to excite an erbium ion in the EDF 242 and form an invertedpopulation. When the WDM optical signal is input to the EDF 242 wherethe inverted population is formed, the EDF 24Z in which stimulatedemission is induced by the WDM optical signal, amplifies the WDM opticalsignal. Thus, the EDF 242 is excited bi-directionally. Since the LDs 244and 245 are pump light sources for the EDF 242, oscillation wavelengthsthereof are set at an excitation wavelength of the EDF 242, for example,1480 nm, 980 nm, and so on.

An optical fiber amplifier doped with an erbium element is used as anamplifier in the optical amplifying part 240 in this embodiment, but arare earth element may be chosen according to an amplifying band of theoptical amplifying part 240. As rare earth elements for amplifying otherbands, for, example, neodymium (Nd), praseodymium (Pr), thulium (Tm),and so on are known. Thus, the present invention is not limited to theuse of any specific rare earth element.

The LD drive circuit 246 outputs control signals to the LD 244 and theLD 245 respectively. The LD drive circuit 246 adjusts, for example,element temperature of the LDs 244 and 245 to stabilize the oscillationwavelengths of the laser light. Furthermore, the LD drive circuit 246adjusts, for example, drive current of the LDs 244 and 245 based on acontrol signal of the CPU 231 to control optical power of the laserlight, whereby a gain of the optical amplifying part 240 is controlled.There are many different ways to appropriately drive a laser diode, andthe present invention is not limited to the specific way described here.

The optical amplifying part 240 in FIG. 17 has a structure in which theWDM optical signal is amplified by one stage of the erbium-doped opticalfiber amplifier, but the optical amplifying part 240 is not limited tothis structure. For example, the optical amplifying part 240 may have atwo-stage structure, including, for example, a first optical amplifierfor amplifying light, an optical attenuating part for attenuating thelight output from the first optical amplifier, and a second opticalamplifier for amplifying the light output from the optical attenuator.In the optical amplifying part 240 thus structured, a gain as a functionof wavelength, especially a gain slope of the optical amplifying part240, can be adjusted in the first and the second optical amplifiers andan output level of the optical amplifying part 240 can be adjusted inthe optical attenuator. As an optical attenuator for adjusting theoutput level, an optical variable attenuator which is able to attenuatean input light to thereby output it and to change attenuation amountthereof is suitable. The optical variable attenuator might by a typewhich adjusts the attenuation amount by adding a magnetooptical crystalbetween the input light and an output light, adding a polarizer to anoutput side of the magnetooptical crystal, and applying a magnetic fieldonto the magnetooptical crystal to change the strength of the magneticfield.

The WDM optical signal output from the optical amplifying part 240 isinput to an optical multiplexer/demultiplexer (hereinafter abbreviatedto ‘W-CPL’) 224 for multiplexing the wavelengths of two input lights.

Meanwhile, an OS 229, which, for example, has the same structure as thatof an optical sender as previously described, generates an opticalsignal modulated by supervisory information (hereinafter abbreviated to‘OSC’). The supervisory information is information such as maintenanceinformation and status information which are necessary in operating theoptical communication system and includes at least an output level ofthe WDM optical signal output from the optical transmitting station 101.The OSC is set, for example, on a shorter wavelength side than ch. 1,which is a minimum channel of the WDM optical signal. However, thepresent invention is not limited to the OSC being set at such awavelength. For example, the OSC may be set on a longer wave side thanch. m, which is a maximum channel of the WDM optical signal.

The OSC generated in the OS 229 is input to the W-CPL 224. The W-CPL 224multiplexes wavelengths of the WDM optical signal output from theoptical amplifying part 240 and the OSC. The WDM optical signal whosewavelength is multiplexed with that of the OSC is input to the CPL 225to be divided into two signals. The one of the divided signals is inputto a PD 227 and the other is sent out to an optical transmission line102-1 as an output of the optical transmitting station 101 to betransmitted to an optical repeater station 104-1 on a next stage.

The PD 227 photoelectrically converts input light and an output thereofis converted to a digital signal in an A/D 230 and thereafter input tothe CPU 231. The PD 227 detects the output level of the WDM opticalsignal output from the optical transmitting station 101.

The CPU 231 is connected to the A/Ds 228 and 230, the LD drive circuit246, a memory 232, and the OS 229 to transmit/receive signals to/formeach of the circuits. Based on an output of the A/D 228 and an output ofthe A/D 230, the CPU 231 controls the optical amplifying part 240 toobtain a fixed gain and controls it to obtain a fixed output. The CPU231 detects the output level of the WDM optical signal output from theoptical transmitting station 101 based on the output of the A/D 230,notifies the OS 229 of information on the output level, accommodates theinformation on the output level in the OSC, and notifies the opticalrepeater station 104-1 on the next stage of it.

The memory 232 is a storage circuit such as, for example, asemiconductor memory. Memory 232 stores control programs for controllingthe optical amplifying part 240 and various data.

Incidentally, an optical isolator may be provided, for example, in someplace between the MUX 222 and the CPL 225. For example, it may beprovided between the EDF 242 and the CPL 241 or between the EDF 242 andthe CPL 243. The optical isolator is an optical component which passeslight therethrough only in one direction and it can be structured, forexample, by disposing a Faraday rotator between two polarizers which aredeviated by 45 degrees from each other. The optical isolator preventsreflected light which occurs in a connecting part and so on between eachoptical component in the optical transmitting station 101 frompropagating without limit. Particularly, when the reflected lightreturns to the semiconductor laser, the semiconductor laser is inducedby the reflected light with various phases and amplitudes, whereby anoscillation mode thereof is changed and a noise is generated. Theseadverse effects can be prevented by the optical isolator.

Next, the structure of the optical repeater station 104 is explained.

FIG. 18 is a diagram showing the structure of the optical repeaterstation in the optical communication system. In FIG. 18, a WDM opticalsignal transmitted, for example, along the optical transmission line102-1, is input to a CPL 253 via a W-CPL 251. Pump light output from apump light source unit 270 is input to the W-CPL 251.

The pump light source unit 270, which is an optical circuit forsupplying the optical transmission line 102-1 with the pump light usedfor distributed Raman amplification, is comprised of, for example,W-CPLs 272 and 273, polarization beam splitters (hereinafter abbreviatedto ‘PBS’) 274 to 276, LDs 277 to 282, and an LD drive circuit 283.

Laser light output from the LD 277 and laser light output from the LD278 are input to the PBS 274 and polarized/synthesized to be laser lighthaving linear polarization constituents which cross at right angles witheach other. Similarly, laser light output from the LD 279 and laserlight output from the LD 280 are input to the PBS 275 to bepolarized/synthesized and laser light output from the LD 281 and laserlight output from the LD 282 are input to the PBS 276 to bepolarized/synthesized.

The laser light polarized/synthesized in the PBS 275 and the laser lightpolarized/synthesized in the PBS 276 are input to the W-CPL 273 andwavelengths thereof are multiplexed. The laser light whose wavelengthsare multiplied in the W-CPL 273 and the laser light which ispolarized/synthesized in the PD 274 are input to the W-CPL 272 andwavelengths thereof are multiplexed.

The laser light whose wavelengths are multiplexed in the W-CPL 272 isinput as the pump light to the optical transmission line 102-1 via theW-CPL 251 and Raman-amplifies the WDM optical with the opticaltransmission line 102-1 serving as an amplifying medium.

Here, for example, as for oscillation wavelengths of the LDs 277 to 282,the LD 277 is set at a wavelength of 1422.0 nm, the LD 278 at awavelength of 1426.0 nm, the LD 279 at a wavelength of 1433.0 nm, the LD280 at a wavelength of 1437.0 nm, the LD 281 at a wavelength of 1459.5nm, and the LD 282 at a wavelength of 1463.5 nm respectively in order toRaman-amplify the WDM optical signal in which each optical signal isarranged at 1529 nm to 1569 nm. When considering a gain as a function ofwavelength in a case in which the Raman-amplification is performed bylaser light with one wavelength, a gain as a function of wavelength in acase in which the Raman amplification is performed by the pump lightincluding six wavelengths can be formed in a substantially linear shapeat 1529 nm to 1569 nm by setting the oscillation wavelengths of the LDs277 to 282 at these wavelengths. Of course, these are only examplewavelengths, and the present invention is not limited to any specificwavelengths.

The LD drive circuit 283 outputs control signals to the LDs 277 to 282respectively. The LD drive circuit 283 adjusts the element temperatureof the LDs 277 to 282 to stabilize the oscillation wavelengths of thelaser light. Furthermore, the LD drive circuit 283 adjusts drive currentof the LDs 277 to 282 based on a control signal of a CPU 261 to controloptical power of the laser light so that a gain of the Ramanamplification is controlled.

Incidentally, in FIG. 18, the pump light source unit 270 is comprised,for example, of six LDs 277 to 282 in order to obtain necessary pumplight power and obtain a necessary gain as a function of wavelength, butthe number of LDs may be determined according to the necessary pumplight power and gain as a function of wavelength.

The WDM optical signal input to the CPL 253 is divided into two signals.One of the divided signals is input to a PD 256 and the other is inputto a W-CPL 261.

The PD 256 photoelectrically converts the input light and an outputthereof is input to the CPU 261 after converted to a digital signal inan A/D 258. The PD 256 detects an output level of the WDM optical signalinput to the optical repeater station.

The W-CPL 261 demultiplexes wavelengths of the OSC and th WDM opticalsignal to output the OSC to an optical receiver (hereinafter abbreviatedto ‘OR’) 263 and output the WDM optical signal to an optical signalprocessing unit 262. Therefore, a cutoff wavelength of the W-CPL 261 isset between wavelength bands of the OSC and the WDM optical signal. TheOR 263 receives and processes the OSC to take out the supervisoryinformation from the OSC and notifies the CPU 261 of the supervisoryinformation. Thereby, the CPU 261 can obtain an output level of theoptical transmitting station 101 on the preceding stage.

The optical signal processing unit 262 amplifies the WDM optical signaland/or adds/drops a predetermined optical signal (channel) to/from theWDM optical signal according to a function required by the opticalrepeater station 104-1.

The structure in the case of amplifying the WDM optical signal is thesame as the structure of the optical amplifying part 240 which isdescribed referring to FIG. 17, and therefore, the explanation thereofis omitted.

The structure in the case of adding/dropping is such that, for, example,in order to drop a predetermined optical signal to be dropped, a CPL fordividing the WDM optical into two, an optical signal rejecting part forrejecting a predetermined optical signal from the WDM optical signalwhich is output from the CPL, a W-CPL for adding an optical signal to beadded to the WDM optical signal which is output from the optical signalrejecting part (with the predetermined optical signal being rejected)are provided. The optical signal rejecting part is structured byconnecting in cascade optical filters, for example, fiber gratingfilters (hereinafter abbreviated to ‘FBG’), in the number correspondingto a predetermined number of optical signals to be dropped. Reflectedwavelength bands of the FBGs correspond to wavelengths of channels to bedropped respectively.

Incidentally, an acoustooptical tunable filter (AOTF) may be used, forexample, as the optical signal rejecting part. The acoustoopticaltunable filter is an optical component which induces a refractive indexchange in an optical waveguide by an acoustooptical effect and rotates apolarization state of light which propagates through the opticalwaveguide to separate/select wavelength.

The structure in the case of carrying out both of the functions can beobtained by connecting each of the structures in cascade.

The WDM optical signal output from the optical signal processing unit262 is input to a W-CPL 254.

Meanwhile, an OS 259 generates the OSC and thereafter, inputs the OSC tothe W-CPL 254. The OSC includes at least an output level of the WDMoptical signal output from the optical repeater station 104-1.

The W-CPL 254 multiplexes wavelengths of the WDM optical signal outputfrom the optical signal processing unit 262 and the OSC. The WDM opticalsignal to which the wavelength of the OSC is multiplexed is input to aCPL 255 to be divided into two signals. One of the divided signals isinput to a PD 257 and the other is sent out to an optical transmissionline 102-2 as an output of the optical repeater station 104-1 to betransmitted to an optical repeater station 104-2 on a next stage.

The PD 257 photoelectrically converts the input light and an outputthereof is input to the CPU 261, after being converted to a digitalsignal in an A/D 260. The PD 257 detects the output level of the WDMoptical signal output from the optical repeater station 104-1.

The CPU 261 is connected to the A/Ds 258 and 260, the optical signalprocessing unit 262, a memory 1262, the OR 263, the OS 259 and the LDdrive circuit 283 in the pump light source unit 270 to transmit/receivesignals to/from these circuits. The CPU 261 controls the LD drivecircuit 283 based on the output level of the optical transmittingstation 101 on the preceding stage which is obtained from the OSC in theOR 263 while referring to an output of the A/D 258 so that opticallevels at both ends of the optical transmission line 102-1 become equalto each other or an optical level at an output end of the opticaltransmission line 102-1 becomes a predetermined value. The CPU 261strengthens the pump light power by increasing the drive current of theLDs 277 to 282 when an input level does not reach the output level ofthe optical transmitting station 101 on the preceding stage, and weakensthe pump light power by decreasing the drive current of the LDs 277 to282 when the input level exceeds the output level of the opticaltransmitting station 101 on the preceding stage. The CPU 261 detectsfrom the output of the A/D 260 the output level of the WDM opticalsignal which is output from the optical repeater station 1041 as a localstation, notifies the OS 259 of information about this output level,accommodates the information about the output level in the OSC, andnotifies the optical repeater station 104-2 on the next stage of it

The memory 1262 stores, for example, control programs for controllingthe pump light source unit 270 and various data.

In this way, in the optical communication system as shown in FIG. 3, theWDM optical signal is subsequently repeated in the plural opticalrepeater stations 104 from the optical transmitting station 101 andreceived in the optical receiving station 103. Here, each of the opticalrepeater stations 104 obtains an output level of the optical repeaterstation 104 on a preceding stage from the OSC, uses it for controllingthe pump light source unit 270 of the own repeater station 104,accommodates the output level of the own repeater station 104 again inthe OSC, and thereafter, notifies an optical repeater station 104 on anext station of it.

Next, the structure of the optical receiving station 103 is explained.

FIG. 19 is a diagram showing the structure of the optical receivingstation in the optical communication system.

In FIG. 19, a WDM optical signal transmitted to the optical receivingstation 103 from an optical repeater station 104 a on a preceding stagevia an optical transmission line 102-a+1 is input to a CPL 292 via aW-CPL 291. To the W-CPL 291, pump light output from a pump light sourceunit 297 is input. The pump light source unit 297 is an optical circuitfor supplying the optical transmission line 102-a+1 with the pump lightfor distributed Raman amplification and has the same structure, forexample, as that of the pump light source unit 270, and therefore, theexplanation thereof is omitted.

The WDM optical signal input to the CPL 292 is divided into two signals.One of the divided signals is input to a PD 295 and the other is inputto a W-CPL 293.

The PD 295 photoelectrically converts the input light and an outputthereof is input to a CPU 299 after being converted to a digital signalin an A/D 298. The PD 295 detects an output level of the WDM opticalsignal input to the optical receiving station 103.

The W-CPL 293 demultiplexes wavelengths of an OSC and the WDM opticalsignal to output the OSC to an OR 296 and output the WDM optical signalto an optical amplifying part 294. Therefore, a cutoff wavelength of theW-CPL 293 is set between the wavelength of the OSC and the wavelengthband of the WDM optical signal. The OR 296 receives and processes theOSC to take out supervisory information from the OSC and notifies theCPU 299 of the supervisory information. Thereby, the CPU 299 can obtainthe output level of the optical transmitting station 104-a on apreceding stage.

The optical amplifying part 294 is an optical circuit for amplifying theWDM optical signal to a predetermined optical level and has the samestructure as that of the optical amplifying part 240 which is describedreferring to FIG. 17 and therefore, the explanation thereof is omitted.

The WDM optical signal output from the optical amplifying part 294 isinput to an optical demultiplexer (hereinafter abbreviated to ‘DEMUX’)301 for demultiplexing light to each wavelength. The DEMUX 301demultiplexes the WDM optical signal to each optical signalcorresponding to each channel. The demultiplexed optical signalscorresponding to respective channels are input to ORs 302-1 to 302-mrespectively to be received/processed. Each of the ORs 302 is composedof, for example, an optical receiving part such as a photodiode, anequalization amplifier for equalizing an output of the optical receivingpart a timing circuit for extracting a timing from an output of theequalization amplifier, and a discriminating circuit for taking out asignal from the output of the equalization amplifier at the timing ofthe timing circuit.

The CPU 299 is connected to the A/D 298, the pump light source unit 297,the optical amplifying part 294, and a memory 303 to transmit/receivesignals to/from these devices. The CPU 299 controls the LD drive circuit283 in the pump light source unit 270 in based on an output of the A/D298.

The memory 303 stores control programs for controlling the pump lightsource unit 297 and various data.

As an example, a dielectric multilayered film filter which is one ofinterference filters, an arrayed waveguide grating, and so on can beutilized as the MUX, the DEMUX or the W-CPL

As shown in FIG. 4A, each of the optical transmission lines whichconnects each section between the optical transmitting station 101, eachof the optical repeater stations 104, and the optical receiving station103 includes, in each section, the first optical transmission line102-L1 with a small characteristic value, the second opticaltransmission line 102-L2 with a large characteristic value, and thethird optical transmission line 102-L3 with a small characteristic valuein sequence from a transmitting side to a receiving side. As the firstoptical transmission line 102-L1 and the third optical transmission line102-L3, for example, a single mode optical fiber can be used and, as thesecond optical transmission line 102-L2, for example, a wavelengthdispersion shift optical fiber can be used.

Here, mode field diameters are different from each other due todifference in characteristic values.

FIGS. 20A, 20B and 20C are views showing examples of mode conversionsplicing.

In FIG. 20A, an optical fiber with a small characteristic value has alarger mode field diameter (r1) than a mode field diameter (r5) of anoptical fiber with a large characteristic value (r1>r5). Therefore, whenthe first optical transmission line 102-L1 (the third opticaltransmission line 102-L3) and the second optical transmission line102-L2 are simply connected, a large amount of connection loss is causeddue to difference in mode field diameter.

Herein, as shown in FIG. 20B, a single or a plural of, for example,three optical transmission lines with different mode field diameters forconnecting parts are prepared and with the use of optical transmissionlines 102-a to 102-c the mode field diameters are changed step by stepto connect the optical transmission lines. Alternately, as shown in FIG.20C, they may be connected with each other with the mode field diametersbeing changed step by step by fusing the connecting parts.

In order to compensate the wavelength dispersion of the opticaltransmission line 102, it is suitable that an absolute value of awavelength dispersion slope d₁ of the first optical transmission line102-L1 (the third optical transmission line 102-L3) and an absolutevalue of a wavelength dispersion slope d₂ of the second opticaltransmission line 102-L2 are made almost equal to each other so that thewavelength dispersion slope of the optical transmission line 102 becomesalmost zero.

Next, the structure of a bi-directional optical communication system isexplained.

FIGS. 22A and 22B are diagrams showing the structure of thebi-directional optical communication system. Referring now to FIGS. 22Aand 22B, the bi-directional optical communication system includesoptical transmitting/receiving stations 111-A and 111-B for generating aWDM optical signal of a plurality of waves in the number of m andreceiving/processing the transmitted WDM optical signal, optical fibercables 112-1 through 112-a+1, each of which is comprised of opticalfibers (see 122-L1, 122-L2 and 122-L3 in FIG. 22B) for transmitting anupward WDM optical signal therethrough between the opticaltransmitting/receiving stations 111-A and 111-B and serving as opticalamplifying media, and optical fibers (see 123-L1, 123-L2 and 123-L3 inFIG. 22B) for transmitting a downward WDM optical signal therethroughand serving as optical amplifying media.

Furthermore, in the bi-directional optical communication system, opticalrepeater stations (such as optical repeater stations 114-1 to 114-a inFIG. 22A) are connected between each of the optical fiber cables. Theplural optical repeater stations 114-1 to 114-a are provided betweeneach of the optical fiber cables 112-1 to 112-a+1.

Each optical repeater station 114-1 to 114-a typically includes a pumplight source for supplying pump light for distributed opticalamplification. For example, as shown in FIG. 22B, optical repeaterstations 114-A and 114-B are adjacent to each other along thetransmission line. Optical repeater station 114-A includes pump lightsource units 270-Aa and 270-Ab for supplying cables with pump light fordistributed optical amplification. Similarly, optical repeater station114-B includes pump light source units 270-Ba and 270-Bb for supplyingcables with pump light for distributed optical amplification. The pumplight source units 270 would typically also be provided in the opticaltransmitting/receiving stations 111-A and 111-B.

Each of the optical transmission/receiving stations 111-A and 111-B canbe structured, for example, by combining the optical transmittingstation 101 and the optical receiving station 103 as described above.Each of the optical repeater stations 114-a through 114-a can bestructured, for example, by combining two optical repeater stations 104as described above.

Each of the optical fiber cable accommodates therein a plurality ofoptical fiber core fibers being tied together and is basicallystructured by including the optical fiber core fibers, high tensilematerial, and an outer cover. The optical fiber core fibers are opticalfibers covered with protective material such as, for example, nylon. Thehigh tensile material prevents extension of the optical fibers which iscaused by tension when the optical fiber cable is laid so that excessiveextension does not occur in the optical fibers. Various optical fibercables are being developed from the viewpoint of transmissioncharacteristics, laying operability, and connecting operability, and,for example, a nylon core fiber unit cable, a loose tube cable, a slotcable, and a ribbon slot cable are among them.

As shown in FIG. 22B, the optical fiber 122 for transmitting the upwardWDM optical signal therethrough is comprised of a first optical fiber122-L1 which has a small characteristic value, a second optical fiber122-L2 which is connected to the first optical fiber 122-L1 and has alarge characteristic value, and a third optical fiber 122-L3 which isconnected to the second optical fiber 122-L2 and has a smallcharacteristic value. The first optical fiber 122-L1 is connected to anoptical repeater station 114-A which is disposed on the upstream side ofthe transmission direction of the WDM optical signal, and the thirdoptical fiber 122-L3 is connected to an optical repeater station 114-Bwhich is disposed on the downstream side of the transmission directionof the WDM optical signal.

Meanwhile, as shown in FIG. 22B, the optical fiber 123 for transmittingthe downward optical signal is comprised of a first optical fiber 123-L1which has a small characteristic value, a second optical fiber 123-L2which is connected to the first optical fiber 123-L1 and has a largecharacteristic value, and a third optical fiber 123-L3 which isconnected to the second optical fiber 123-L2 and has a smallcharacteristic value. The first optical fiber 123-L1 is connected to theoptical repeater station 114-B which is disposed on the upstream side ofthe transmission direction of the WDM optical signal and the thirdoptical fiber 123-L3 is connected to the optical repeater station 114-Awhich is disposed on the downstream side of the transmission directionof the WDM optical signal.

In the bi-directional optical communication system as described above,the optical fibers 122 and 123 whose characteristic values are larger inmiddle fields thereof than characteristic values in fields other thanthe middle fields are used to transmit a WDM optical signal and toRaman-amplify the WDM optical signal. The wavelength dispersion, thetransmission loss, and the nonlinear optical effect can be compensatedin a well-balanced manner as a whole and the optical SNR can be improvedmost.

Furthermore, due to its symmetric characteristic, a length of theoptical fiber 122-L1 and a length of the optical fiber 123-L3, a lengthof the optical fiber 122-L2 and a length of the optical fiber 123-L2,and a length of the optical fiber 122-L3 and a length of the opticalfiber 123-L1 can be mad equal to each other, respectively, so that theoptical fiber cable can be manufactured easily.

The distributed optical amplifying apparatus, the optical communicationstation, the optical communication system, and the optical fiber cableaccording to the present invention can compensate the wavelengthdispersion, the transmission loss, and the nonlinear optical effect in awell-balanced manner as a whole and also improve the optical SNR most.

Therefore, the transmission distance can be lengthened compared withthat in a conventional art.

According to various of the above embodiments of the present invention,a distributed optical amplifying apparatus includes an optical fiberwhose characteristic value in a middle field thereof is larger thancharacteristic values in fields other than the middle field when a valueof a nonlinear refractive index divided by an effective cross section issupposed to be a characteristic value, and a pump light source forsupplying pump light to the optical fiber.

Further, according to various embodiments of the present invention, adistributed optical amplifying apparatus includes an optical fiber whichis comprised of a first optical fiber which has a first characteristicvalue, a second optical fiber which is connected to the first opticalfiber and has a second characteristic value larger than thecharacteristic value of the first optical fiber, and a third opticalfiber which is connected to the second optical fiber and has a thirdcharacteristic value smaller than the characteristic value of the secondoptical fiber. The characteristic value is a nonlinear refractive indexdivided by an effective cross section. A pump light source supplies pumplight to the optical fiber.

Moreover, according to embodiments of the present invention, an opticalcommunication station includes a processing device for performingpredetermined processing for an optical signal, an optical fiber whichis connected to the processing device and is composed of a first opticalfiber having a first characteristic value, a second optical fiber whichis connected to the first optical fiber and has a second characteristicvalue larger than the characteristic of the first optical fiber, and athird optical fiber which is connected to the second optical fiber andhas a third characteristic value smaller than the characteristic valueof the second optical fiber. The characteristic value is a nonlinearrefractive index divided by an effective cross section. The opticalcommunication station also includes a pump light source supplying pumplight to the optical fiber.

In addition, according to above embodiments of the present invention, anoptical communication system includes a first station and a secondstation for performing predetermined processing for an optical signal.The optical communication system also includes an optical transmissionline for connecting the first station and the second station. Theoptical transmission line is an optical fiber composing of a firstoptical fiber which has a first characteristic value, a second opticalfiber which is connected to the first optical fiber and has a secondcharacteristic value larger than the characteristic value of the firstoptical fiber, and a third optical fiber which is connected to thesecond optical fiber and has a third characteristic value smaller thanthe characteristic value of the second optical fiber. The characteristicvalue is a nonlinear refractive index divided by an effective crosssection. A pump light source supplies pump light to the optical fiber.

According to various embodiments of the present invention, by an opticalfiber cable includes a plurality of optical fibers having characteristicvalues in middle fields thereof larger than characteristic values infields other than the middle fields, wherein the characteristic value isa nonlinear refractive index divided by an effective cross section.

According to embodiments of the present invention, a distributed opticalamplifying apparatus, an optical communication station, an opticalcommunication system, and an optical fiber cable as described above areprovided with an optical fiber having a specific structure as describedabove so that the wavelength dispersion, the transmission loss, and thenonlinear optical effect can be compensated in a well-balanced manner asa whole and the optical SNR can be improved. As a result, transmissiondistance can be lengthened compared with that in the conventional art

Various values or levels are described herein as being almost equal toeach other, thereby indicating that the values or levels aresubstantially equal.

Various embodiments of the present invention relate to a fiber lineincluding a plurality of optical fibers connected together. For example,in various embodiments of the present invention, such as in FIG. 4A, anoptical transmission line 102 includes first, second and third opticalfibers 102-L1, 102-L2 and 102-L3 connected together. The term “fiberline” is simply intended to indicate an optical fiber (such as opticaltransmission line 102) formed of a plurality of optical fibers connectedtogether, and is not limited to being a “transmission” line for anoptical communication system.

Although a few preferred embodiments of the present invention have beenshown and described, it would be appreciated by those skilled in the artthat changes may be made in these embodiments without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. A distributed optical amplifying apparatus comprising: a fiber linecomprising first, second and third optical fibers connected together sothat light travels through the fiber line from the first optical fiber,then through the second optical fiber and then through the third opticalfiber, the first, second and third optical fibers having first, secondand third characteristic values, respectively, the second characteristicvalue being larger than the first characteristic value and the thirdcharacteristic value, the characteristic value of a respective opticalfiber being a nonlinear refractive index of the optical fiber divided byan effective cross section of the optical fiber; and a pump light sourcesupplying pump light to the fiber line to cause Raman amplification tooccur in the fiber line.
 2. The distributed optical amplifying apparatusaccording to claim 1, wherein the first and third characteristic valuesare almost equal.
 3. The distributed optical amplifying apparatusaccording to claim 1, wherein a product of wavelength dispersion of thefirst optical fiber multiplied by wavelength dispersion of the secondoptical fiber is negative in a wavelength band of an optical signaltransmitted through the fiber line, and wavelength dispersion of thethird optical fiber is almost equal to that of the first optical fiber.4. The distributed optical amplifying apparatus according to claim 3,wherein accumulated wavelength dispersion in a wavelength of the opticalsignal transmitted through the fiber line is almost zero at an output ofthe fiber line.
 5. The distributed optical amplifying apparatusaccording to claim 1, wherein a product of a wavelength dispersion slopeof the first optical fiber multiplied by a wavelength dispersion slopeof the second optical fiber is negative in a wavelength band of anoptical signal transmitted through the fiber line and a wavelengthdispersion slope of the third optical fiber is almost equal to that ofthe first optical fiber.
 6. The distributed optical amplifying apparatusaccording to claim 1, wherein an accumulated wavelength dispersion slopein a wavelength of the optical signal transmitted through the fiber lineis in a linear shape relative to distance.
 7. The distributed opticalamplifying apparatus according to claim 5, wherein accumulatedwavelength dispersion in a wavelength of the optical signal transmittedthrough the fiber line is almost zero at an output of the fiber line. 8.The distributed optical amplifying apparatus according to claim 1,wherein the first optical fiber and the third optical fiber are almostthe same in length.
 9. The distributed optical amplifying apparatusaccording to claim 1, wherein a ratio of a sum of a length of the firstoptical fiber and a length of the third optical fiber to a length of thesecond optical fiber is in the range of approximately 1:1 toapproximately 2:1.
 10. The distributed optical amplifying apparatusaccording to claim 1, wherein the fiber line is 50 km or more in length.11. The distributed optical amplifying apparatus according to claim 1,wherein a relative gain of a Raman on/off gain relative to total loss ofthe fiber line is approximately 0.5 up to
 1. 12. An opticalcommunication station comprising: processing means for performingpredetermined processing for an optical signal; a fiber line connectedto said processing means and comprising first, second and third opticalfibers connected together so that the optical signal travels through thefiber line by traveling through the first optical fiber, then throughthe second optical fiber, an then through the third optical fiber, thefirst, second and third optical fibers having first, second and thirdcharacteristic values, respectively, the second characteristic valuebeing larger than the first and third characteristic values, thecharacteristic value of a respective optical fiber being a nonlinearrefractive index of the optical fiber divided by an effective crosssection of the optical fiber; and a pump light source supplying pumplight to the fiber line to cause Raman amplification to occur in thefiber line.
 13. The optical communication station according to claim 12,wherein said processing means is receiving means for receiving theoptical signal.
 14. The optical communication station according to claim12, wherein said processing means is repeating/amplifying means foramplifying the optical signal.
 15. The optical communication accordingto claim 12, wherein said processing means is dropping/adding means fordropping and/or adding an optical signal with a predetermined wavelengthfrom/to a wavelength-division multiplexing optical signal in whichwavelengths of a plural of optical signals with different wavelengthsfrom each other are multiplexed.
 16. An optical communication systemcomprising: an optical transmission line; first and second stationsconnected together through the optical transmission line and performingpredetermined processing of an optical signal transmitted through theoptical transmission line, and a pump light source supplying pump lightto the transmission line to cause Raman amplification to occur in thetransmission line, wherein the transmission line comprises first, secondand third optical fibers connected together so that the optical signaltravels through the transmission line by traveling through the firstoptical fiber, then through the second optical fiber, and then throughthe third optical fiber, the first, second and third optical fibershaving first, second and third characteristic values, respectively, thesecond characteristic value being larger than the first and thirdcharacteristic values, the characteristic value of a respective opticalfiber being a nonlinear refractive index of the optical fiber divided byan effective cross section of the optical fiber.
 17. An opticalcommunication system comprising: first and second transmission lines,each having first and second ends; an optical transmitting stationgenerating an optical signal and providing the generated optical signalto the first end of the first transmission line so that the opticalsignal travels through the first transmission line to the second end ofthe first transmission line; an optical repeater station receiving theoptical signal from the second end of the first transmission line,amplifying the received optical signal, and providing the amplifyingoptical signal to the first end of the second transmission line so thatthe amplified optical signal travels through the second transmissionline to the second end of the second transmission line; and an opticalreceiving station receiving the amplified optical signal from the secondend of the second optical transmission line, wherein at least one of thefirst and second transmission lines comprises first, second and thirdoptical fibers connected together so that the optical signal travelsthrough the respective transmission line by traveling through the firstoptical fiber, then through the second optical fiber, and then throughthe third optical fiber, the first, second and third optical fibershaving first, second and third characteristic values, respectively, thesecond characteristic value being larger than the first and thirdcharacteristic values, the characteristic value of a respective opticalfiber being a nonlinear refractive index of the optical fiber divided byan effective cross section of the optical fiber, pump light beingproviding to the respective transmission line to cause Ramanamplification to occur in the respective transmission line.
 18. Theoptical communication system according to claim 17, wherein the opticalrepeater station comprises dropping/adding means for dropping and/oradding a predetermined optical signal from/to a wavelength-divisionmultiplexing optical signal in which wavelengths of a plural of opticalsignals with different wavelengths from each other are multiplexed. 19.An apparatus comprising: a transmission line comprising first, secondand third optical fibers connected together from an input end to anoutput end of the transmission line s that a signal light travelsthrough the input end, then through the first optical fiber, thenthrough the second optical fiber, then through the third optical fiberand then through the output end, the first, second and third opticalfibers having first, second and third characteristic values,respectively, the second characteristic value being larger than thefirst characteristic value and the third characteristic value, thecharacteristic value of a respective optical fiber being a nonlinearrefractive index of the optical fiber divided by an effective crosssection of the optical fiber, and a pump light source supplying pumplight to the transmission line so that the signal light is amplified byRaman amplification as the signal light travels through the transmissionline.
 20. The apparatus according to claim 19, wherein the first andthird characteristic values are almost equal.
 21. The apparatusaccording to claim 19, wherein a product of wavelength dispersion of thefirst optical fiber multiplied by wavelength dispersion of the secondoptical fiber is negative in a wavelength band of the signal light, andwavelength dispersion of the third optical fiber is almost equal to thatof the first optical fiber.
 22. The apparatus according to claim 21,wherein accumulated wavelength dispersion in a wavelength of the signallight is almost zero at the output end of the transmission line.
 23. Theapparatus according to claim 19, wherein a product of a wavelengthdispersion slope of the first optical fiber multiplied by a wavelengthdispersion slope of the second optical fiber is negative in a wavelengthband of the signal light and a wavelength dispersion slope of the thirdoptical fiber is almost equal to that of the first optical fiber. 24.The apparatus according to claim 23, wherein an accumulated wavelengthdispersion slope in a wavelength of the signal light traveling throughthe transmission line is in a linear shape relative to distance.
 25. Theapparatus according to claim 23, wherein accumulated wavelengthdispersion in a wavelength of the signal light is almost zero at theoutput end of the transmission line.
 26. The apparatus according toclaim 19, wherein the first optical fiber and the third optical fiberare almost the same in length.
 27. The apparatus according to claim 19,wherein a ratio of a sum of a length of the first optical fiber and alength of the third optical fiber to a length of the second opticalfiber is in the range of approximately 1:1 to approximately 2:1.
 28. Theapparatus according to claim 19, wherein the transmission line is 50 kmor more in length.
 29. The apparatus according to claim 19, wherein arelative gain of a Raman on/off gain relative to total loss of thetransmission line is approximately 0.5 up to
 1. 30. An opticalcommunication system comprising: an optical transmission line; first andsecond stations connected together through the optical transmission lineand performing predetermined processing of a signal light transmittedthrough the transmission line, and a pump light source supplying pumplight to the transmission line so that the signal light is amplified byRaman amplification as the signal light travels through the transmissionline, wherein the transmission line comprises first, second and thirdoptical fibers connected together so that the optical signal travelsthrough the transmission line by traveling through the first opticalfiber, then through the second optical fiber, and then through the thirdoptical fiber, the first, second and third optical fibers having first,second and third characteristic values, respectively, the secondcharacteristic value being larger than the first and thirdcharacteristic values, the characteristic value of a respective opticalfiber being a nonlinear refractive index of the optical fiber divided byan effective cross section of the optical fiber.
 31. An opticalcommunication system comprising: first and second transmission lines,each having first and second ends; an optical transmitting stationgenerating an optical signal and providing the generated optical signalto the first end of the first transmission line so that the opticalsignal travels through the first transmission line to the second end ofthe first transmission line; an optical repeater station receiving theoptical signal from the second end of the first transmission line,amplifying the received optical signal, and providing the amplifyingoptical signal to the first end of the second transmission line so thatthe amplified optical signal travels through the second transmissionline to the second end of the second transmission line; and an opticalreceiving station receiving the amplified optical signal from the secondend of the second optical transmission line, wherein at least one of thefirst and second transmission lines comprises. first, second and thirdoptical fibers connected together so that the optical signal travelsthrough the respective transmission line by traveling through the firstoptical fiber, then through the second optical fiber, and then throughthe third optical fiber, the first, second and third optical fibershaving first, second and third characteristic values, respectively, thesecond characteristic value being larger than the first and thirdcharacteristic values, the characteristic value of a respective opticalfiber being a nonlinear refractive index of the optical fiber divided byan effective cross section of the optical fiber, pump light beingsupplied to the respective transmission line so that the optical signalis amplified by Raman amplification as the optical signal travelsthrough the respective transmission line.
 32. The optical communicationsystem according to claim 31, wherein the optical repeater stationcomprises dropping/adding means for dropping and/or adding apredetermined optical signal from/to a wavelength-division multiplexingoptical signal in which wavelengths of a plural of optical signals withdifferent wavelengths from each other are multiplexed.
 33. An apparatuscomprising: a transmission line comprising first, second and thirdoptical fibers, the first optical fiber being connected to the secondoptical fiber and the second optical fiber being connected to the thirdoptical fiber so that a signal light traveling through the transmissionline travels through the first optical fiber, then through the secondoptical fiber, then through the third optical fiber, the first, secondand third optical fibers having first, second and third characteristicvalues, respectively, the second characteristic value being larger thanthe first characteristic value and the third characteristic value, thecharacteristic value of a respective optical fiber being a nonlinearrefractive index of the optical fiber divided by an effective crosssection of the optical fiber; and a pump light source supplying pumplight to the transmission line so that the signal light is amplified byRaman amplification as the signal light travels through at least one ofthe first, second and third optical fibers.
 34. A distributed opticalamplifying apparatus comprising: a fiber line comprising first, secondand third optical fibers connected together so that light travelsthrough the fiber line from the first optical fiber, then through thesecond optical fiber and then through the third optical fiber, thefirst, second and third optical fibers having first, second and thirdcharacteristic values, respectively, the second characteristic valuebeing larger than the first characteristic value and the thirdcharacteristic value, the characteristic value of a respective opticalfiber being a nonlinear refractive index of the optical fiber divided byan effective cross section of the optical fiber; and a pump light sourcesupplying pump light to the fiber line, wherein a product of wavelengthdispersion of the first optical fiber multiplied by wavelengthdispersion of the second optical fiber is negative in a wavelength bandof an optical signal transmitted through the fiber line, and wavelengthdispersion of the third optical fiber is almost equal to that of thefirst optical fiber.
 35. The distributed optical amplifying apparatusaccording to claim 34, wherein accumulated wavelength dispersion in awavelength of the optical signal transmitted through the fiber line isalmost zero at an output of the fiber line.
 36. A distributed opticalamplifying apparatus comprising: a fiber line comprising first, secondand third optical fibers connected together so that light travelsthrough the fiber line from the first optical fiber, then through thesecond optical fiber and then through the third optical fiber, thefirst, second and third optical fibers having first, second and thirdcharacteristic values, respectively, the second characteristic valuebeing larger than the first characteristic value and the thirdcharacteristic value, the characteristic value of a respective opticalfiber being a nonlinear refractive index of the optical fiber divided byan effective cross section of the optical fiber; and a pump light sourcesupplying pump light to the fiber line, wherein a product of awavelength dispersion slope of the first optical fiber multiplied by awavelength dispersion slope of the second optical fiber is negative in awavelength band of an optical signal transmitted through the fiber lineand a wavelength dispersion slope of the third optical fiber is almostequal to that of the first optical fiber.
 37. The distributed opticalamplifying apparatus according to claim 36, wherein an accumulatedwavelength dispersion slope in a wavelength of the optical signaltransmitted through the fiber line is in a linear shape relative todistance.
 38. The distributed optical amplifying apparatus according toclaim 36, wherein accumulated wavelength dispersion in a wavelength ofthe optical signal transmitted through the fiber line is almost zero atan output of the fiber line.
 39. A distributed optical amplifyingapparatus comprising: a fiber line comprising first, second and thirdoptical fibers connected together so that light travels through thefiber line from the first optical fiber, then through the second opticalfiber and then through the third optical fiber, the first, second andthird optical fibers having first, second and third characteristicvalues, respectively, the second characteristic value being larger thanthe first characteristic value and the third characteristic value, thecharacteristic value of a respective optical fiber being a nonlinearrefractive index of the optical fiber divided by an effective crosssection of the optical fiber; and a pump light source supplying pumplight to the fiber line, wherein the fiber line is 50 km or more inlength.
 40. A distributed optical amplifying apparatus comprising: afiber line comprising first, second and third optical fibers connectedtogether so that light travels through the fiber line from the firstoptical fiber, then through the second optical fiber and then throughthe third optical fiber, the first, second and third optical fibershaving first, second and third characteristic values, respectively, thesecond characteristic value being larger than the first characteristicvalue and the third characteristic value, the characteristic value of arespective optical fiber being a nonlinear refractive index of theoptical fiber divided by an effective cross section of the opticalfiber; and a pump light source supplying pump light to the fiber line,wherein a relative gain of a Raman on/off gain relative to total loss ofthe fiber line is approximately 0.5 up to 1.